In recent years considerable attention has been focused on the extensive network or specialized reticular cells of lymphatic tissue germinal centers. The objective of the proposed research is to obtain an integrated picture of the behavior of the reticular cell framework during the genesis, growth and dissociation of germinal centers as a response to primary and secondary antigenic stimulation of the spleen, lymph nodes and thymus of germfree rats. Sufficient evidence is now available to support the contention that there exists a causal relationship between the initial localization of antigen on the surface of specific reticular cells and the development of active germinal centers. Therefore, the cytological features and topographical distribution of reticular cells in the antigenically unstimulated major lymphatic organs of germfree rats will be studied by light and electron microscopy. Attempts will then be made to determine precisely the special sites at which antigen is retained on the reticular cells of these organs with light and electron microscopy, in combination with correlative radioautographic techniques using a radioactively-labeled, pure antigen. These techniques are also expected to provide valuable information about the antigen-retaining reticular cells during germinal center growth and development.